Moisture availability influences the formation and characteristics of earlywood of Pinus tabuliformis more than latewood in northern China

Northern China is experiencing pronounced warming and drying, with potentially dire implications for the health of the region's forests. It is of the highest importance to explore the growth response mechanisms of Pinus tabuliformis Carr., the important natural conifer in northern China, to identify the appropriate forest management practices. We used the xylem anatomical traits in 11 temperate P. tabuliformis forests distributed along a transect in northern China to investigate the potential climate factors driving earlywood and latewood formation, the timing of maximum climate sensitivity, and xylem anatomical adjustments. Our results indicated that relative humidity is the main climate factor limiting P. tabuliformis growth. The water stress resulting from the increase in surface and atmospheric temperature has a significant effect on the formation of earlywood. However, only the increase in surface temperature significantly affected the formation of latewood. Earlywood formation is most sensitive to climate around the 150th day of the year, which is when P. tabuliformis grows most vigorously. The most serious impact of water stress on latewood formation occurs around the 200th day, i.e., when both tem-peratures and P. tabuliformis growth are at their highest. Climate-driven adjustments in tree-ring anatomical structure are found primarily in earlywood rather than in latewood. In a continuously warming climate, the adjustment of xylem anatomical structure of P. tabuliformis will balance between hydraulic efficiency and safety and differ in dry and wet areas. Frequent severe droughts in dry areas can enhance hydraulic safety and induce P. tabuliformis to form smaller lumens and thicker cell walls (higher rTSR) to survive. P. tabuliformis in wet areas can improve hydraulic efficiency by forming larger lumens and thinner cell walls (lower rTSR) to face moderate droughts. Considering more frequent droughts in the future, forest managers should pay enough attention to the moisture conditions when earlywood formation occurs.